Tìm nguyên hàm
$\int{{x\sqrt{{1+{{x}^{2}}}}}}dx.$
A. $\displaystyle \frac{1}{2}{{\left( {\sqrt{{1+{{x}^{2}}}}} \right)}^{2}}+C.$
B. $\displaystyle \frac{1}{3}{{\left( {\sqrt{{1+{{x}^{2}}}}} \right)}^{3}}+C.$
C. $\frac{{{{x}^{2}}}}{2}{{\left( {\sqrt{{1+{{x}^{2}}}}} \right)}^{2}}+C.$
D. $\displaystyle \frac{1}{3}{{\left( {\sqrt{{1+{{x}^{2}}}}} \right)}^{2}}+C.$